1. Field of the Invention
The present invention relates to an image processing apparatus having a function of reconstructing a three-dimensional (3D) image from projection images that are captured from two directions, or a function of generating projection image data from volume data relating to a configuration of a fine tubular object such as a blood vessel within a subject.
2. Description of the Related Art
There is known a technique of reconstructing a 3D image from projection images captured from two directions in order to visualize a running of, typically, a blood vessel, as shown in FIG. 11 and FIG. 14. To implement this technique, it is necessary to designate a correspondency between characteristic points of the images. For example, as shown in FIG. 13, it is understood that a part, which is projected at a point A on a lateral image (Lateral), is present somewhere on a line B within a 3D space. However, it is not possible to specify the part at one point. The line B is projected in another direction on a line C on a frontal image (Frontal), for instance. Thus, the part, which is projected at the point A, corresponds to a given position on the line C on the frontal image.
If the operator designates a point (characteristic point) A on the lateral image as an anatomically characteristic part and designates a point (corresponding point), which is associated with same part as the characteristic point A, on the line C on the frontal image, the position of the characteristic part on the line B can be specified. In short, in order to specify a 3D position, it is necessary to designate corresponding points on two-directional images in association with the same part.
Thus, the operator is required to perform a work to designate corresponding points on two-directional images by means of a pointer such as a mouse. By increasing the number of corresponding points, the precision of the 3D image is enhanced. When a 3D image of blood vessels, which branch in a complex fashion, is to be acquired, many corresponding points need to be designated.
Typical examples of the method of designating such corresponding points are shown in FIG. 15A and FIG. 15B. In FIG. 15A, corresponding points are alternately designated on images captured in two directions. In FIG. 15B, all corresponding points are first designated on one image, and then all associated corresponding points are designated on the other image.
Practically, it is very time-consuming to designate several-ten, in some cases, several-hundred corresponding points. In either of the above-described two methods, errors tend to occur in establishing the correspondency of the corresponding points.
There have been an increasing number of opportunities in which configurations of fine tubular objects, typically, blood vessels are displayed three-dimensionally. Practically, a projection image (re-projection image) is generated from 3D image data having depth information relating to blood vessels, and shading is added to the image to achieve three-dimensional visualization.
In fact, however, it is very difficult to understand the direction of running of blood vessels in the direction of image projection.